1. Field of the Invention
The present invention relates to a fitting apparatus for fitting two workpieces to each other using a robot while performing compliance control or force control.
2. Description of the Related Art
In carrying out assembly using a robot, it is required to position a workpiece in place on a table while at the same time moving another workpiece accurately to the place with the robot. However, between the workpiece held by the robot and the workpiece fixed to the table, there exists a position error and an orientation error, which sometimes adversely affects assembly, or especially, the fitting of the two workpieces by the robot. In order to solve this problem, a fitting apparatus for fitting two workpieces to each other generally uses a RCC (remote center compliance) mechanism in which a spring or a damper is interposed between the robot arm and the robot gripper to thereby provide a mechanically elastic motion of the gripper mounted at the forward end of the robot arm, so that one workpiece held in the gripper can be fitted smoothly with the other workpiece by automatically changing the position and the orientation of the one workpiece held in the gripper even in the presence of some position error or orientation error. Further, as described in Japanese Examined Patent Publication No. 4-43744, Japanese Examined Patent Publication No. 6-83976, Japanese Unexamined Patent Publication No. 5-38637 and Japanese Patent No. 2604929, a compliance control method or force control method can be used in which the robot is operated as if there exists a spring or a damper between the robot arm and the robot gripper by controlling the operation of the robot arm to thereby obtain the same effect as the RCC mechanism.
In the conventional compliance control or force control, one of the two workpieces to be fitted to each other is held by the gripper of the robot arm and pressed against the other workpiece to thereby carry out the fitting operation, while the force and moment exerting on the workpiece held by the gripper are detected and the operation of the robot arm is controlled and corrected so as to bring the detected force and moment to a target value. In correcting the operation of the robot arm for each control period, the difference between the detected force and the target force and the difference between the detected moment and the target moment are multiplied by a parameter called a force control gain to thereby calculate a command velocity and a command angular velocity for the robot arm. Based on the command velocity and the command angular velocity thus calculated, the workpiece held by the gripper is translated and rotated. Therefore, the force control gain exhibits the force control performance and a larger value of the force control gain can achieve a quicker correction of the position and orientation of the workpiece held by the gripper.
However, as the force control gain is increased, the vibration of the robot arm and the vibration due to the contact between the workpieces are liable to be amplified and the operation of the robot arm is easily oscillated. Therefore, in the conventional compliance control or force control, the force control gain can be increased only to a value determined by the oscillation limit, and in the system having an initial error in the position and orientation of the workpieces and the robot arm, the error correction requires a vast amount of time.